Photomask for exposure to optical near-field, method of controlling optical near-field intensity distribution using the same, pattern preparing method and pattern preparing apparatus

ABSTRACT

A photomask for exposure to optical near-field has a micro-aperture and is adapted to expose an object of exposure to light by using light seeping out from the micro-aperture. The photomask also has periodically arranged recesses or projections so as to uniformize the optical near-field intensity distribution in the micro-aperture.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a photomask for exposure to optical near-field, to a method of controlling an optical near-field intensity distribution using such a photomask, to a pattern preparing method and also to a pattern preparing apparatus.

[0003] 2. Related Background Art

[0004] With the current advancement of technology brought forth to realize semiconductor memories having a higher memory capacity and CPUs that operate at very high speed and show an enhanced degree of integration, photolithography is inevitably required to accommodate micro-processing operations with smaller dimensions. Generally, the dimensional limit of micro-processing of a photolithography apparatus approximately corresponds to the wavelength of the light source to be used for photolithography. Therefore, a near-ultraviolet laser is often used as light source for a photolithography apparatus in order to reduce the dimensional limit. Thus, currently, photolithography apparatus are feasible for micro-processing of about 0.1 μm.

[0005] While the dimensional limit of micro-processing is reduced for photolithography, there are still a number of problems to be solved for photolithography if it is to be used for micro-processing of 0.1 μm and less. For example, a light source of a shorter wavelength may be needed. Then, lenses that can be used for such a short wavelength may have to be developed.

[0006] Near-field exposure methods have been proposed as a possible solution for the problems. For example, U.S. Pat. No. 6,171,730 proposes an exposure method and an exposure apparatus that utilizes an elastically deformable mask having a micro-aperture pattern of an opening with a width not greater than 100 nm on the front surface and made of an elastic material so as to be deformable along the normal line relative to the mask surface. The method and the apparatus as disclosed in the above-cited patent document are very excellent and have contributed greatly to the technological field to which the present invention relates.

[0007] However, when g rays having a wavelength of 436 nm or i rays having a wavelength of 365 nm are used for a micro-aperture pattern with a width not greater than 100 nm as disclosed in U.S. Pat. No. 6,171,730, the width of the openings is less than a half of the wavelength.

[0008] In an operation of near-field exposure using a near-field formed by light by way of a micro-aperture on a metal film, the optical near-field intensity distribution can be different from the contour of the aperture.

[0009] Meanwhile, U.S. Pat. No. 6,236,033 proposes a photolithography mask formed on a metal film and having an aperture and a surface profile that undulates periodically so as to interact with surface plasmon modes and intensify the transmission of light through the aperture for the purpose of transferring an image. However, while the above-cited patent document proposes to boost light obtained through the aperture, it does not propose to weaken light and uniformize the intensity distribution of light.

SUMMARY OF THE INVENTION

[0010] The present invention provides a photomask for exposure to optical near-field, a method of controlling an optical near-field intensity distribution using such a photomask, a pattern preparing method and also a pattern preparing apparatus as will be described hereinafter.

[0011] In an aspect of the invention, there is provided a photomask for exposure to optical near-field having a micro-aperture and adapted to expose an object of exposure to light by using light seeping out from the micro-aperture, said mask having periodically arranged recesses or projections so as to uniformize the optical near-field intensity distribution in said micro-aperture.

[0012] For the purpose of the invention, the aperture width of said micro-aperture may be not greater than ½ of the wavelength of light from the light source.

[0013] For the purpose of the invention, said optical near-field intensity distribution may be controlled by way of the positions and/or the sizes of said recesses or projections relative to said micro-aperture.

[0014] Said recesses or projections may be arranged periodically relative to said micro-aperture and said optical near-field intensity distribution may be controlled by way of the extent of shift of the pitch and/or the phase of period.

[0015] Said pitch of period may be made shorter than the intra-medium wavelength of light for exposure in the mask base member of said photomask.

[0016] For the purpose of the invention, it may be so arranged that said micro-aperture includes a micro-aperture group of a plurality of micro-apertures and, said photomask has said recesses or projections in the vicinity of an area of weak optical near-field intensity that is produced when light for exposure is applied in said micro-aperture group.

[0017] Alternatively, it may be so arranged that said micro-aperture includes a micro-aperture group of a plurality of micro-apertures and said photomask has said recesses or projections in the vicinity of an area of strong optical near-field intensity that is produced when light for exposure is applied in said micro-aperture group.

[0018] For the purpose of the invention, it may be so arranged that said micro-aperture includes a micro-aperture group of a plurality of micro-apertures and said photomask has at least a first recess or projection in the vicinity of an area of weak optical near-field intensity that is produced when light for exposure is applied in said micro-aperture group and at least a second recess or projection in the vicinity of an area of strong optical near-field intensity that is produced when light for exposure is applied in said micro-aperture group, said first recess or projection and said second recess or projection being different from each other in terms of relative position and/or size relative to said micro-aperture.

[0019] Still alternatively, it may be so arranged that said micro-aperture includes a micro-aperture group of a plurality of micro-apertures and said photomask has first recesses or projections arranged periodically in the vicinity of an area of weak optical near-field intensity that is produced when light for exposure is applied in said micro-aperture group and second recesses or projections arranged periodically in the vicinity of an area of strong optical near-field intensity that is produced when light for exposure is applied in said micro-aperture group, said first recess or projection and said second recess or projection being different from each other in terms of the pitch and/or phase of period.

[0020] For the purpose of the invention, it may be so arranged that said micro-aperture is slit-shaped and said photomask has said area of weak optical near-field intensity in the vicinity of each of the ends of said slit-shaped micro-aperture, said recess or projection being formed in the vicinity of the each of the ends of said slit-shaped micro-aperture.

[0021] For the purpose of the invention, it may be so arranged that said micro-aperture shows an isolated pattern having a longitudinal dimension and a transversal dimension smaller than the wavelength and said area of weak optical near-field intensity is located in the vicinity of the isolated pattern, whereas said recess or projection is formed in the vicinity of the isolated pattern.

[0022] For the purpose of the invention, it may be so arranged that the phase of period forms a discontinued section in the periodically arranged recesses or projections and said discontinued section is arranged in the vicinity of said micro-aperture.

[0023] For the purpose of the invention, it may be so arranged that said micro-aperture includes a micro-aperture group of a plurality of micro-apertures and said photomask is provided with a first periodic structure having a discontinued section in said phase of period in the vicinity of an area of weak optical near-field intensity that is produced when light for exposure is applied in said micro-aperture group and a second periodic structure having a discontinued section in said phase of period in the vicinity of an area of strong optical near-field intensity that is produced when light for exposure is applied in said micro-aperture group, the amount of discontinuation of phase being different between said first periodic structure and said second periodic structure.

[0024] In another aspect of the invention, there is provided a method of controlling an optical near-field intensity distribution using a photomask for exposure to optical near-field according to the invention and adapted to control the intensity distribution of optical near-field by adjusting the coupled relation of light being propagated through said micro-aperture and plasomon polaritons on the surface of said photomask.

[0025] In still another aspect of the invention, there is provided a pattern preparing method comprising: arranging a photomask for exposure to optical near-field on a substrate to be processed, said substrate carrying a photoresist film thereon with a thickness not greater than the wavelength of light from a light source; irradiating light for exposure from said light source onto said photoresist film by way of said photomask; and forming/transferring a latent image on said photoresist film on the basis of the aperture pattern formed in said photomask by controlling the intensity distribution of optical near-field.

[0026] In still another aspect of the invention, there is provided a pattern preparing apparatus comprising: a stage adapted to carry thereon a photomask for exposure to optical near-field according to the invention; a light source for exposure; a specimen table adapted to carry thereon a substrate to be processed, said substrate carrying a photoresist film thereon with a thickness not greater than the wavelength of light from said light source; and a distance control means for controlling the distance between the substrate to be processed and the photomask.

[0027] Thus, according to the invention, it is possible not only to control the intensity distribution of optical near-field at will but it is also possible to realize a photomask for exposure to optical near-field that can control the intensity distribution so as to make it uniform, a method of controlling the intensity distribution of optical near-field by using such a photomask, a pattern preparing method and a pattern preparing apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 is a schematic illustration of the first embodiment of photomask for exposure to optical near-field according to the invention;

[0029]FIG. 2 is a schematic illustration of a photomask for exposure to optical near-field obtained by modifying the first embodiment of the invention;

[0030]FIG. 3 is a schematic illustration of the second embodiment of photomask for exposure to optical near-field according to the invention;

[0031]FIG. 4 is a schematic illustration of the third embodiment of photomask for exposure to optical near-field according to the invention;

[0032]FIGS. 5A, 5B, 5C, 5D, 5E, 5F, 5G and 5H are numerical computation models of optical near-field in a near-field micro-aperture produced by periodically arranged recesses and projections provided for the purpose of explaining the first embodiment of the invention;

[0033]FIG. 6 is a graph obtained by computationally determining the distribution of optical near-field provided for the purpose of explaining the first embodiment of the invention;

[0034]FIG. 7 is a contour graph illustrating the pitch lambda (nm) of the periodic structure and the intensity of light immediately below the aperture relative to the phase as provided for the purpose of explaining the first embodiment of the invention;

[0035]FIG. 8 is a schematic illustration of the pattern preparing apparatus used in Example 1; and

[0036]FIGS. 9A, 9B, 9C and 9D are schematic illustrations of the pattern preparing method used in Example 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] Now, the present invention will be described in greater detail by referring to the accompanying drawings.

First Embodiment

[0038]FIG. 1 is an enlarged partial view of the first embodiment of photomask for exposure to optical near-field according to the invention. The photomask comprises a base member 101 that is transparent relative to the wavelength of light from a light source to be used for exposure and a metal thin film 102 that blocks light from the light source. The thin film has a thickness between 40 and 200 nm.

[0039] The base member 101 is also a thin film having a thickness between 0.1 and 100 μm that is supported along the outer peripheral section thereof by a support member (not shown). The metal film 102 has a slit-shaped aperture 103. The width of the slit is smaller than half of the wavelength of light from the light source, whereas the length of the slit is greater than the wavelength of light from the light source.

[0040] Recesses/projections 104 are formed periodically in the vicinity of each of the ends of the slit of the metal film 102 in a direction perpendicular to the longitudinal direction of the slit (in the direction of AA′ in FIG. 1).

[0041] The mask is made to tightly adhere to the thin film resist applied to a substrate. Then, light is irradiated onto it in a direction perpendicular to it so that the pattern is exposed to light. Now, the behavior of light in the vicinity of the mask will be discussed in detail below.

[0042] When light for exposure is irradiated onto the mask from the side of the base member, optical near-field is produced in the vicinity of the aperture on the front surface of the mask (the surface at the lower side in the cross sectional view of FIG. 1).

[0043] The periodic recesses/projections 104 at each of the ends of the slit of this embodiment are designed to boost the intensity of light at a part of the slit located near the recesses/projections so as to minimize the difference in the optical near-field intensity over the entire area of the mask aperture and realize a quasi-flat light intensity distribution pattern.

[0044] The distributed optical near-field is weak in the vicinity of each of the ends of the slit when there are no periodic recesses/projections 104 for the reason as described below.

[0045] Assume that light is irradiated onto the mask of FIG. 1 without controlling polarization of light. The incident light can be divided into a parallel component whose electric field vector is parallel to the slit and a perpendicular component whose electric field vector is perpendicular to the slit. The slit does not allow the component of light whose electric field vector is parallel to the slit (and hence parallel to the metal surfaces, or the lateral surfaces, of the slit) to pass through it. On the other hand, the component of light whose electric field vector is perpendicular to the slit is propagated through the slit and produces optical near-field on the front surface of the mask. However, the propagation loss of light is increased in the slit because the electric field vector runs in parallel with the metal surface at each of the ends of the slit. Such loss affects the range from the ends of the slit to the wavelength of light from the light source.

[0046] The concept of controlling the intensity of light by means of periodic recesses/projections 104 arranged near the slit is based on the principle as described below. The periodic recesses/projections 104 control coupling of interface plasmon polaritons and light that is being propagated through the slit on the surface of the metal film (the interface of the metal film and the base member or the interface of the photoresist and the metal film when the metal film tightly adheres to the photoresist).

[0047] More specifically, interface plasmon polaritons are scattered by the periodic recesses/projections to give rise to a standing wave. The intensity of optical near-field is boosted if the position of the slit is found near a loop of the standing wave, whereas it is weakened if the position of the slit is found near a node of the standing wave.

[0048] This effect will be described further by way of specific numerical computation models as shown in FIGS. 5A through 5H. Chromium (Cr, complex refractive index; 1.775-4.035i) is used as metal and a value of 436 nm is assumed for the wavelength in vacuum of light from a light source. For the purpose of simplicity, 2D models where the refractive index of the base member is equal to 1 and the slit has an infinite length are used.

[0049] Firstly, two parameters of the pitch Λ and the phase Φ of recesses/projection that are used for describing the profile of periodic recesses/projections in a case where there is no slit will be explained by referring to FIGS. 5A through 5D. A slit is to be arranged along the dotted chain line in each of FIGS. 5A through 5D. However, the phase and the pitch are defined on the basis of the profile of the recesses/projections before arranging a slit. The pitch Λ is the distance for a period of a recess and a projection as shown in FIG. 5A. Phase Φ=0 is defined for the periodic structure of FIG. 5A where the phase does not show any discontinuity at the axis of symmetry and the latter is the center of a recess. On the other hand, phase Φ=π/4 is defined for the periodic structure of FIG. 5B and phase Φ=π/2 defined for that of FIG. 5C, phase Φ=π is defined for that of FIG. 5D.

[0050] While the phase Φ is equal to π also at the left side of the slit and there is no discontinuity of phase in the periodic recesses/projections of FIG. 5D, the periodic structure of FIG. 5D differs from that of FIG. 5A in terms of phase.

[0051]FIGS. 5E through 5H show periodic structures where a slit is arranged at the axis of symmetry and correspond respectively to FIGS. 5A through 5D. Note, however, that the recess where the slit is arranged is buried in each of FIGS. 5E and 5F. Two recesses are arranged at each side of the slit.

[0052] Then, light is irradiated onto the structure from the side where recesses are formed periodically and the intensity distribution of optical near-field is determined by numerical analysis, using the FDTD method. The slit has a width of 80 nm and the light shielding film has a thickness of 60 nm.

[0053]FIG. 6 shows the computationally obtained values of the intensity of light (squared amplitude of the electric filed) at positions separated from the mask surface by 10 nm. In FIG. 6, the solid line indicates the intensity distribution of optical near-field when no periodic recesses are arranged. It is provided as reference for comparison. On the other hand, the intensity of light is increased by about 1.5 times relative to the reference value when recesses are arranged periodically with a pitch of Λ=410 nm and a phase of Φ=π/2 (dotted chain line), whereas it is reduced to about 0.6 times relative to the reference value when recesses are arranged periodically with a pitch of Λ=340 nm and a phase of Φ=0 (broken line). The pitch that is used to increase the intensity of light is close to the wavelength of surface plasmons along the vacuum/Cr interface or

λspp=λ ₀[(ε_(m)+ε_(d))/(ε_(m)×ε_(d))]^(1/2)=428.4 nm

[0054] but does not completely agrees with the latter. In the above formula, ε_(m) represents the dielectric constant of Cr and ε_(d) represents the dielectric constant of the dielectric substance involved (vacuum in the computation models).

[0055]FIG. 7 is a contour graph illustrating the pitch lambda (nm) of the periodic structure as shown in, any of FIGS. 5A through 5H and the intensity of light immediately below the aperture relative to the phase (deg), where intensity of light is made equal to 1 when there is no periodic structure. As in the case of the numerical computation models of FIGS. 5A through 5H, chromium (Cr, complex refractive index; 1.775-4.035i) is used as metal and a value of 436 nm is assumed for the wavelength in vacuum of light from a light source. Again, for the purpose of simplicity, 2D models where the refractive index of the base member is equal to 1 and the slit has an infinite length are used.

[0056] For the effect of intensifying optical near-field by means of periodic recesses/projections as used in the above computation models to appear only in regions surrounding the respective ends of the slit where the intensity of optical near-field falls, it is necessary to form periodic recesses/projections only in the vicinity of each of the ends of the slit of the mask. When the refractive index of the base material of the photoresist that is tightly adhering to the mask is not 1 as used in the above described numerical computation models but n, it is necessary to multiply the above cited pitch Λ of arrangement of periodic recesses/projections by 1/n to obtain Λ/n.

[0057] In this way, it is possible to obtain a distribution of intensity that approximately reflects the shape of the slit and hence to form a resist pattern that approximately reflects the shape of the slit by appropriately controlling the optical near-field intensity distribution, using a photomask on which the pitch and/or the phase of the periodically arranged recesses are appropriately selected. While periodic recesses/projections are formed only on the surface of the metal film that is irradiated with light in the above description, periodic recesses/projections may alternatively be formed on the front surface of the mask, although the effect will be weakened to a certain extent. Still alternatively, periodic recesses/projections may be formed on the opposite surfaces of the metal film.

[0058] While recesses are formed in the above-described instance, it may be needless to say that a similar effect is obtained by forming periodic projections.

[0059]FIG. 2 shows a modified arrangement of periodic recesses, where square recesses are formed around a single micro-aperture to obtain a mask pattern that can effectively increase the intensity of optical near-field intensity due to the arrangement of periodic recesses.

Second Embodiment

[0060]FIG. 3 is an enlarged partial view of the second embodiment of photomask for exposure to optical near-field according to the invention. With this embodiment, the intensity of optical near-field can be reduced by means of periodic recesses for the purpose of controlling the intensity of optical near-field.

[0061] The mask pattern of FIG. 3 is formed by arranging periodic recesses 304 in the vicinity of a middle part of slit-shaped aperture 303, where the intensity of optical near-field is high. In this case, both the intensity of light propagated through the slit and that of optical near-field on the surface of the mask are reduced by shifting the pitch and the phase of the periodic recesses 304.

[0062] With this arrangement again, it is possible to control the optical near-field intensity distribution in the vicinity of the slit and obtain an exposure pattern that approximately reflects the shape of the slit.

[0063] In case of controlling the optical near-field intensity distribution not only for a single slit, but the respective optical near-field intensity distributions for a plurality of slits whose shapes may vary from each other, so as to obtain a resist pattern, as the whole mask pattern, having a desired profile, the use of such a pattern that can reduce the intensity of optical near-field is effective.

Third Embodiment

[0064]FIG. 4 is an enlarged partial view of the third embodiment of photomask for exposure to optical near-field according to the invention. With this embodiment, the intensity of optical near-field can be reduced in certain regions and, at the same time, increased in other regions on the mask by using a plurality of micro-apertures.

[0065] The mask pattern illustrated in FIG. 4 is formed by arranging a group of slit-shaped apertures 403 that run in parallel with each other. Recesses 404 are arranged periodically in the vicinity of each of the ends of the group of slit-shaped apertures 403 for the purpose of intensifying optical near-field, whereas recesses 405 are arranged periodically in the vicinity of a central part of the group of slits for the purpose of reducing the intensity of optical near-field.

[0066] With the periodic arrangement of recesses, it is possible to control not only the optical near-field intensity distribution in a single slit but also the difference in the intensities of optical near-field among the plurality of slits so as to expose a resist pattern that corresponds to the shapes of the slit to light.

[0067] Now, the present invention will be described further by way of examples.

EXAMPLE 1

[0068]FIG. 8 is a schematic illustration of the pattern preparing (exposure) apparatus used in Example 1.

[0069] In FIG. 8, reference symbol 801 denotes a photomask according to the invention as described above by way of embodiments. The front surface (the lower surface in FIG. 8) of the photomask 801 is directed to the outside of pressure adjustable container 805, whereas the rear surface (the upper surface in FIG. 8) of the photomask 801 is directed to the inside of the pressure adjustable container 805. The internal pressure of the pressure adjustable container 805 can be adjusted by a pressure adjusting means 813.

[0070] The object of exposure in this example was a substrate 806 carrying a resist film 807 that was formed on the surface thereof. The resist 807/substrate 806 was placed on a stage 808 and the stage 808 was driven so as to align the substrate 806 relative to the photomask 801 in intra-planar two-dimensional directions of the mask. Subsequently, the stage 808 was driven in a direction along the normal line relative to the mask surface so as to make the photomask 801 tightly adhere to the resist 807 on the substrate 806.

[0071] Then, the evanescent light exposure mask 801 was made to tightly adhere to the resist 807 on the substrate 806 by adjusting the internal pressure of the pressure adjustable container 805 by the pressure adjusting means 813 until the gap between the front surface of the mask 801 and the corresponding surface of the resist 807 became not greater than 100 nm over the entire surface area of the mask 801.

[0072] Thereafter, light for exposure 810 emitted from light source 809 was collimated by a collimator lens 811 and then made to pass through a glass window 812 so as to be introduced into the pressure adjustable container 805 and irradiated onto the evanescent light exposure mask 801 from the rear surface thereof (upper surface in FIG. 8). The resist 807 was exposed to optical near-field produced in the vicinity of the micro-apertures on the front surface of the photomask 801. It was possible to transfer the different patterns 804 on the photomask on the substrate 806 as patterns of clear contrast by using the photomask 801.

EXAMPLE 2

[0073]FIGS. 9A through 9D are schematic illustrations of the pattern preparing method including a single buffer layer used in Example 2.

[0074]FIG. 9A shows the photomask and the object of exposure of this example. The photomask 904 is a photomask according to the invention as described above by way of embodiments.

[0075] In this example, positive type photoresist was applied onto an Si substrate 901 by means of a spin coater. Subsequently, the photoresist was heated at 120° C. for 30 minutes to produce the first layer 902, which had a film thickness of 400 nm.

[0076] Thereafter, negative type photoresist that contained Si was applied onto the first layer 902 and pre-baked to produce the second layer 903, which had a film thickness of 40 nm. Thus, the photoresist had a two-layered structure.

[0077] Then, the Si substrate 901 carrying the two-layered photoresist thereon, which was formed as a result of the application process, was brought close to the photomask 904 by means of the exposure apparatus as shown in FIG. 8 and pressure was applied thereto in order to make the resist layer 903 tightly adhere to the photomask 904.

[0078] Light for exposure 905 (the photomask was prepared to meet the wavelength of light) was irradiated onto the photoresist layer 903 on the substrate 901 by way of the photomask. In other words, the photoresist layer 903 was exposed to light by way of the patterns on the photomask 904 (FIG. 9B).

[0079] Subsequently, the photomask was removed from the surface of the photoresist layer 903, which was then subjected to a development process and a post-baking process. As a result, the patterns on the photomask were transferred as resist patterns (FIG. 9C).

[0080] Thereafter, the first photoresist layer 902 was subjected to an oxygen reactive ion etching process, using the patterns formed on the second photoresist layer 903 as etching mask (FIG. 9D).

[0081] The Si contained in the second photoresist layer 903 was oxidized in the oxygen reactive ion etching process to raise the etching-resistance of the layer.

[0082] As a result of following the above-described procedure, it was possible to transfer the different patterns on the photomask on the substrate 901 as patterns of clear contrast. 

What is claimed is:
 1. A photomask for exposure to optical near-field having a micro-aperture and adapted to expose an object of exposure to light by using light seeping out from the micro-aperture, said mask having periodically arranged recesses or projections so as to uniformize the optical near-field intensity distribution in said micro-aperture.
 2. A photomask according to claim 1, wherein the aperture width of said micro-aperture is not greater than ½ of the wavelength of light from the light source.
 3. A photomask according to claim 1, wherein said optical near-field intensity distribution is controlled by way of the positions and/or the sizes of said recesses or projections relative to said micro-aperture.
 4. A photomask according to claim 1, wherein said recesses or projections are arranged periodically relative to said micro-aperture and said optical near-field intensity distribution is controlled by way of the extent of shift of the pitch and/or the phase of period.
 5. A photomask according to claim 4, wherein said pitch of period is made shorter than the intra-medium wavelength of light for exposure in the mask base member of said photomask.
 6. A photomask according to claim 1, wherein said micro-aperture includes a micro-aperture group of a plurality of micro-apertures and said photomask has said recesses or projections in the vicinity of an area of weak optical near-field intensity that is produced when light for exposure is applied in said micro-aperture group.
 7. A photomask according to claim 1, wherein said micro-aperture includes a micro-aperture group of a plurality of micro-apertures and said photomask has said recesses or projections in the vicinity of an area of strong optical near-field intensity that is produced when light for exposure is applied in said micro-aperture group.
 8. A photomask according to claim 1, wherein said micro-aperture includes a micro-aperture group of a plurality of micro-apertures and said photomask has at least a first recess or projection in the vicinity of an area of weak optical near-field intensity that is produced when light for exposure is applied in said micro-aperture group and at least a second recess or projection in the vicinity of an area of strong optical near-field intensity that is produced when light for exposure is applied in said micro-aperture group, said first recess or projection and said second recess or projection being different from each other in terms of relative position and/or size relative to said micro-aperture.
 9. A photomask according to claim 1, wherein said micro-aperture includes a micro-aperture group of a plurality of micro-apertures and said photomask has first recesses or projections arranged periodically in the vicinity of an area of weak optical near-field intensity that is produced when light for exposure is applied in said micro-aperture group and second recesses or projections arranged periodically in the vicinity of an area of strong optical near-field intensity that is produced when light for exposure is applied in said micro-aperture group, said first recess or projection and said second recess or projection being different from each other in terms of the pitch and/or phase of period.
 10. A photomask according to claim 6, wherein said micro-aperture is slit-shaped and said photomask has said area of weak optical near-field intensity in the vicinity of each of the ends of said slit-shaped micro-aperture, said recess or projection being formed in the vicinity of the each of the ends of said slit-shaped micro-aperture.
 11. A photomask according to claim 6, wherein said micro-aperture shows an isolated pattern having a longitudinal dimension and a transversal dimension smaller than the wavelength and said area of weak optical near-field intensity is located in the vicinity of the isolated pattern, whereas said recess or projection is formed in the vicinity of the isolated pattern.
 12. A photomask according to claim 4, wherein the phase of period forms a discontinued section in the periodically arranged recesses or projections and said discontinued section is arranged in the vicinity of said micro-aperture.
 13. A photomask according to claim 12, wherein said pitch of period is made shorter than the intra-medium wavelength of light for exposure in the mask base member of said photomask.
 14. A photomask according to claim 4, wherein said micro-aperture includes a micro-aperture group of a plurality of micro-apertures and said photomask is provided with a first periodic structure having a discontinued section in said phase of period in the vicinity of an area of weak optical near-field intensity that is produced when light for exposure is applied in said micro-aperture group and a second periodic structure having a discontinued section in said phase of period in the vicinity of an area of strong optical near-field intensity that is produced when light for exposure is applied in said micro-aperture group, the amount of discontinuation of phase being different between said first periodic structure and said second periodic structure.
 15. A photomask according to claim 14, wherein said pitch of period is made shorter than the intra-medium wavelength of light for exposure in the mask base member of said photomask.
 16. A method of controlling an optical near-field intensity distribution using a photomask for exposure to optical near-field according to claim 1 and adapted to control the intensity distribution of optical near-field by adjusting the coupled relation of light being propagated through said micro-aperture and plasmon polaritons on the surface of said photomask.
 17. A pattern preparing method comprising: arranging a photomask for exposure to optical near-field according to claim 1 on a substrate to be processed, said substrate carrying a photoresist film thereon with a thickness not greater than the wavelength of light from a light source; irradiating light for exposure from said light source onto said photoresist film by way of said photomask; and forming/transferring a latent image on said photoresist film on the basis of the aperture pattern formed in said photomask by controlling the intensity distribution of optical near-field.
 18. A pattern preparing apparatus comprising: a stage adapted to carry thereon a photomask for exposure to optical near-field according to claim 1; a light source for exposure; a specimen table adapted to carry thereon a substrate to be processed, said substrate carrying a photoresist film thereon with a thickness not greater than the wavelength of light from said light source; and distance control means for controlling the distance between the substrate to be processed and the photomask. 